This invention relates to a system for preventing, retarding or reversing the decomposition of silicon carbide refractory or ceramic articles during high temperature sintering.
Silicon carbide has several physical and chemical properties which make it an excellent material for high temperature, structural uses. Mechanically, silicon carbide is a hard, rigid, brittle solid which does not yield to applied stresses even at temperatures approaching its decomposition temperature. Because of its high thermal conductivity, silicon carbide is an excellent material for heat exchangers, muffle type furnaces, crucibles, gas-turbine engines and retorts in the carbothermic production and distillation of zinc. Silicon carbide is also used in electrical resistance elements, ceramic tiles, boilers, around tapping holes, in heat treating, annealing and forging furnaces, in gas producers, and in other places where strength at high temperatures, shock resistance and slag resistance are required. Properties associated with silicon carbide refractory and ceramic materials are superior strength, high elastic modulus, high fracture toughness, corrosion resistance, abrasion resistance, thermal shock resistance, and low specific gravity.
Silicon carbide refractory or ceramic materials are generally sintered at temperatures above 1900.degree. C. so that the silicon carbide articles will develop desirable physical and chemical properties such as high strength, high density and low chemical reactivity. A reducing or inert atmosphere is generally used for sintering silicon carbide to prevent formation of compounds which may have undesirable physical or chemical properties. Electric kilns are typically used to sinter silicon carbide ceramic or refractory materials under controlled atmospheres, but these tend to be energy inefficient and slow. In the case of a kiln equipped with graphite heating elements, the voltage can be controlled and the kiln can be heated to fairly high temperatures, yet there are several disadvantages: (1) The heating elements have a limited size, complex shape and must be kept under a strictly controlled atmosphere to maintain a long life; and (2) Furnace size is limited and it is difficult to achieve a uniform temperature in this type of kiln because the heating elements provide only radiant heat. Because of radiant heat transfer, as well as a size limit for heating elements, the kiln has a poor load density, limited productivity, and a poor energy efficiency.
Plasma arc technology has recently been applied to the production of refractory and ceramic materials to reduce the furnace energy requirements and retention times. However, plasma technology has generally only been used for the fusion of high temperature materials and not for sintering or reaction sintering. This is because the required sintering temperature for most ceramic or refractory materials is usually less than 2500.degree. C., whereas the average temperature of gases heated through a plasma arc column is above about 4000.degree. C. For instance, alpha silicon carbide is generally sintered at temperatures of between 1900.degree. C. to 2350.degree. C. At temperatures above around 2150.degree. C., silicon carbide decomposes into silicon gas and solid carbon. The carbon may then react further with the silicon carbide and silicon gas to form other vapor species, such as SiC.sub.2 and Si.sub.2 C. This decomposition of silicon carbide could result in substantial shrinkage of the article being fired, as well as an undesirable change in surface chemistry.
Plasma arc fired gases differ greatly from ordinary furnace heated gases in that they become ionized and contain electrically charged particles capable of transferring electricity and heat; or, as in the case of nitrogen, become dissociated and highly reactive. For example, nitrogen plasma gas dissociates into a highly reactive mixture of N.sub.2 -molecules, N-atoms, N.sup.+ -ions and electrons. This dissociation or ionization greatly increases the reaction rates for sintering ceramic or refractory materials. Nitrogen, for example, which dissociates at around 5000.degree. C. and 1 atmosphere pressure, would not dissociate under the normal furnace sintering conditions of around 1500.degree. C.-2000.degree. C. Thus, the use of plasma gases results in a highly reactive environment, which greatly increases the reaction sintering rate.
However, this highly reactive plasma environment also increases the decomposition of the green body because of the buoyant forces involved in convective heat transfer which increase the flow of the gases in the furnace. These gases sweep away the decomposition products, allowing the decomposition reactions to proceed. In the case of silicon carbide, silicon is continually stripped from the surface of the green body, resulting in a decreased density and undesirable surface chemistry.